<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            width: 100px;
            height: 100px;
            background-color: red!important;
        }
        #one{
            background-color: royalblue;
        }
        .one{
            background-color: skyblue;
        }
        div#one{
            background-color: tomato;
        }
    </style>
</head>
<body>
    <div id="one" class="one"></div>
</body>
</html>